LUNYOV, A. Spectral functions of the simplest even order ordinary differential operator. Methods of Functional Analysis and Topology, [S. l.], v. 19, n. 4, p. 319–326, 2013. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/556. Acesso em: 4 feb. 2026.